refactor: run Spark tests fully in Docker with bridge network

Better approach than mixing host and container networks.

Changes to docker-compose.yml:
- Remove 'network_mode: host' from spark-tests container
- Add spark-tests to seaweedfs-spark bridge network
- Update SEAWEEDFS_FILER_HOST from 'localhost' to 'seaweedfs-filer'
- Add depends_on to ensure services are healthy before tests
- Update volume publicUrl from 'localhost:8080' to 'seaweedfs-volume:8080'

Changes to workflow:
- Remove separate build and test steps
- Run tests via 'docker compose up spark-tests'
- Use --abort-on-container-exit and --exit-code-from for proper exit codes
- Simpler: one step instead of two

Benefits:
✓ All components use Docker DNS (seaweedfs-master, seaweedfs-volume, seaweedfs-filer)
✓ No host/container network split or DNS resolution issues
✓ Consistent with how other SeaweedFS integration tests work
✓ Tests are fully containerized and reproducible
✓ Volume server accessible via seaweedfs-volume:8080 for all clients
✓ Automatic volume creation works (master can reach volume via gRPC)
✓ Data writes work (Spark can reach volume via Docker network)

This matches the architecture of other integration tests and is cleaner.

test/java/spark/docker-compose.yml

@@ -27,7 +27,7 @@ services:
     ports:
       - "8080:8080"
       - "18080:18080"
-    command: "volume -mserver=seaweedfs-master:9333 -ip=seaweedfs-volume -ip.bind=0.0.0.0 -port=8080 -port.grpc=18080 -publicUrl=localhost:8080 -max=100 -dir=/data -preStopSeconds=1"
+    command: "volume -mserver=seaweedfs-master:9333 -ip=seaweedfs-volume -ip.bind=0.0.0.0 -port=8080 -port.grpc=18080 -publicUrl=seaweedfs-volume:8080 -max=100 -dir=/data -preStopSeconds=1"
     volumes:
       - seaweedfs-volume-data:/data
     depends_on:
@@ -69,18 +69,22 @@ services:
   spark-tests:
     image: maven:3.9-eclipse-temurin-17
     container_name: seaweedfs-spark-tests
-    network_mode: "host"
     volumes:
       - .:/workspace
       - ~/.m2:/root/.m2
     working_dir: /workspace
     environment:
       - SEAWEEDFS_TEST_ENABLED=true
-      - SEAWEEDFS_FILER_HOST=localhost
+      - SEAWEEDFS_FILER_HOST=seaweedfs-filer
       - SEAWEEDFS_FILER_PORT=8888
       - SEAWEEDFS_FILER_GRPC_PORT=18888
       - HADOOP_HOME=/tmp
     command: sh -c "sleep 30 && mvn test"
+    depends_on:
+      seaweedfs-filer:
+        condition: service_healthy
+    networks:
+      - seaweedfs-spark
     mem_limit: 4g
     cpus: 2